Process for making resistors



96 w. M. KQHRING 3,048,914

PROCESS FOR MAKING RESISTOR-S Filed Sept. 21, 1959 23 FIG. I

gm m; as m 22 E 4 \j 2 a F G 5 INVENTOR.

WILBUR M. KOHRING ATTORNEYS United States Patent Ofiice 3,048,914Patented Aug. 14, 1962 3,048,914 PROCESS FUR MAKING RESISTORS Wilbur M.Kohring, 3318 W. 159th St., Cleveland 11, Ohio Filed Sept. 21, 1959,Ser. No. 841,317 1 Claim. (Cl. 29155.63)

The invention relates in general to resistors and more particularly toprotective tubular casings for resistors and the process for makingsame.

An object of my invention is to provide a resistor with a protectivetubular casing.

Another object of my invention is to provide a resistor With aprotective coating and a protective tubular casing.

Another object of my invention is to provide a means of sealing the openends of the tubular casing containing a resistor element.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claim, taken in conjunctionwith the accompanying drawings, in which:

' FIGURE 1 shows a longitudinal view of a resistance unit embodying thefeatures of my invention, partly in section;

FIGURES 2 and 3 show the steps by which my invention is constructed; thecoatings and end-caps are exaggerated in thickness;

FIGURE 4 shows a section of a hollow tubular casing; and

FIGURE 5 shows a convenient way of sealing the open ends of the tubularcasing containing aresistor element.

With reference to FIGURE 1, my invention comprises a non-conducting rod26 preferably porcelain or steatite, a thin current-conducting film 21deposited on the surface of the rod 20, end-caps 22 having terminalwires 23 electrically connected to the thin current-conducting film 21,a protective coating 25 mounted on the currentconducting surface, and atubular casing 26 mounted around the protective coating 25.

The rod 20 may be made of suitable material of a ceramic nature such asporcelain or steatite, upon which the film 21 may adhere. The film 21 isvery thin and is exaggerated in thickness in the drawings. The thincurrent-conducting film 2-1 is preferably carbon, or metal, or it may bea mixture of carbon and metal, and may also include sulphur.

The next general series of steps in my process is to connect theend-caps 22 to the end portions of the thin film 21 by depositing anelectrical substance between the current-conducting film and theend-caps 22 and pressing the end-caps 22 over the electrical substance.The resistor element may or may not be spiraled at 24 depending upon theresistance value desired.

The next general series of steps consist of providing a coating and atubular casing for the resistor element, indicated respectively by thereference characters 25 and 26. The coating 25 is deposited on theresistor element and the casing 26 is mounted around the coating 25.

The coating 25 comprises a silicone material preferably a siliconeresin, and is referred to in the trade as a heat resistant,water-repellent silicone electrical varnish. More specifically, thevarnish is a phenyl-methylsilicone resin. In the silicone molecularstructure, the silicon alternates with an oxygen atom so that thesilicon atoms are not bonded to each other. In a phenylmethyl-siliconeresin, one methyl group and one phenyl group are bonded to each siliconatom. I find that a silicone resin made by Dow Corning Corporation,Midland, Michigan, and sold under a designation number 994 varnish issatisfactory for my invention. The

coating 25 is preferably deposited on the current-conducting film 21 byspraying the phenyl-methyl-silicone thereon, and then baking theresistor element with the phenyl-methyl-silicone deposited thereon for aduration of approximately one-half hour at about 400 F. to provide asemi-cured coating.

The casing 26 is in the form of a tube and may be made of a ceramic,steatite, porcelain, or a glass material.

The tube 26 is slightly longer and slightly larger in diameter than theresistor element. The tube 26 may be slid easily over the resistorelement and when completely slid over the resistor, it covers theresistor element as shown in FIGURE 1. The open end portions of the tubeare extended farther than the end-caps 22, whereby a sealing material 28may be deposited in the space provided between the end-caps 22 and theextended tube portions to completely enclose the resistor element.

The sealing material 28 is made of a mixture of the followingingredients; a powder glass, a silicone resin and a silicone varnish.The powdered glass that I use in my invention is made by grinding brokenscrap glass. The glass is ground so fine that a person can run his handthrough it and will not receive any scratches.

The silicone resin that I mix with the powdered glass is preferably apolysiloxane silicone resin and contains dimethylsiloxane,methylsiloxane, phenylsiloxane, and diphenylsiloxane, and is prepared bythe co-hydrolysis and co-condensation of a mixture containingdimethyldichlorosilane, methyltrichlorosilane, phenyltrichlorosilane,and diphenyldichloroxosilane. I find that a polysiloxane resin suitablefor my invention may be obtained from Union Carbide Corporation, NewYork, N.Y., Silicone Division and sold under a designation number R-620.

' The silicone varnish that I use in mixing with powdered glass andsilicone resin, is a low temperature curing silicone varnish. It is acompanion product to the polysiloxane resin designation number R620. Itcontains 60% silicone resin by weight in a solvent xylene. I find that asilicone varnish suitable for my invention may be obtained from UnionCarbide Corporation, New York, N.Y., Silicone Division and sold under adesignation number XR622.

In making the sealing material, I mix the powder glass with the siliconeresin R-620. In mixing the two together, I do not necessarily use adefinite ratio to determine how much I should use of each, but instead Ijust mix enough polysiloxane resin to moisten each particle of powderglass to obtain a glass-silicone mixture. In order to make theglass-silicone mixture more appliable or workable, I mix XR622 into theglass-silicone mixture to obtain a workable sealing material. Here againI do not use any definite amount of silicone varnish, but I add justenough to make the glass-silicone mixture appliable.

Even though a definite amount of each is not required I generally mix inabout 50% by volume of powdered glass with 50% by volume of polysiloxaneresin. Then I generally mix in about 20% by volume of silicone varnishinto about by volume of the glass-silicone mixture.

With reference to FIGURE 5, the diagram shows a resistor carrier 30constituting a convenient way of applying the sealing material 28 intoan open end portion of the resistor element. The resistor carrier 30 maybe made of any suitable material or any shape or size. The main purposeis to hold a resistance element with a ceramic tube 26 thereon andhaving the upper end of the tube extending farther than the end-caps 22.The resistor element may be held in place by extending the bottom leadterminals 23 into a hole 30. The reference character 3-1 shows ashoulder whereby the lower endcap of a resistance element may restthereupon. Another use for the shoulder 31 is to provide a means ofcentering the resistor element inside of the tube, whereby both ends ofthe tube extend an equal distance from the end-caps 22.

The sealing material 28 may be applied into the upper open end portionby means of any suitable tool, preferably a spatula. After I seal theupper end portions, I allow the resistor to set on the resistor carrier30 anywhere from 2 to 24 hours. Then I bake the resistor elementcontaining the sealed end portion for about one hour at approximately400 F., with the resistor element still mounted on the carrier 30.

Next in the series of steps, I seal the lower end portion of the tubecontaining a resistor element in the same manner as I sealed the top endportion. The lower end portion of the resistor element that was holdingit in place during the first operation, is the upper end during thesecond operation, and is sealed accordingly.

During the actual operation, a plurality of resistors carriers 30 areused in order to provide for an efficient and economical resistorsealing process. In this invention, the polysiloxane resin R-620 and thesilicone varnish 994 may be interchangeably used, as they both havesimilar chemical and physical properties.

Although this invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:

The process of providing a protective casing on the surface of aresistor element comprising a non-conductive element having a currentconductive film thereon with hollow terminal end-caps surrounding andelectrically connected to the film on the ends of said element,

said end-caps having a larger external diameter than that of the filmbetween said end caps, said process comprising the steps of providing aphenyl-methyl-silicone resin, spraying said silicone resin on said filmbetween said end-caps to provide a coating thereon, then baking saidresistor element with said coating thereon at approximately 400 F. forabout one-half hour to cure said coating, providing a tubular casinghaving an internal diameter slightly larger than the external diameterof said end-caps and sliding said casing over said end-caps with theopen end portions of said casing extending beyond said end-caps and withsaid end-caps fitting closely within the internal surface of said casingand concentrically supporting said internal surface of said casingaround said film at a circumferential clearance space therefrom, sealingthe open end portions of said casing to enclose said resistor elementtherein, the steps of sealing including the following method, providinga powder glass material, a polysiloxane resin, and a silicone varnish,mixing about fifty percent by volume of powder glass with about sameamount of polysiloxane resin to obtain a glass-silicone mixture, mixingin 20 percent by volume of said silicone varnish with about percent byvolume of said glass-silicone mixture to obtain a sealing material,applying said sealing material in said open end portions of said casingagainst said closely fitting end-caps so as to block said sealingmaterial from contacting said film to enclose said resistor elementtherein, air setting said sealing material for about twenty-four hoursand then baking said sealing material in said end portions atapproximately 400 F., for approximately one hour to seal said resistorelement in said casing.

References Cited in the fileof this patent UNITED STATES PATENTS2,397,568 Seaman Apr. 2, 1946 2,460,795 Warrick Feb. 1, 1949 2,660,653Berkelhamer Nov. 24, 1953 2,742,551 Kohring Apr. 17, 1956

1. THE PROCESS OF PROVIDING A PROTECTIVE CASING ON THE SURFACE OF ARESISTOR ELEMENT COMPRISING A NON-CONDUCTIVE ELEMENT HAVING A CURRENTCONDUCTIVE FILM THEREON WITH HOLLOW TERMINAL END-CAPS SURROUNDING ANDELECTRICALLY CONNECTED TO THE FILM ON THE ENDS OF SAID ELEMENT, SAIDEND-CAPS HAVING A LARGER EXTERNAL DIAMETER THAN THAT OF THE FILM BETWEENSAID END CAPS, SAID PROCESS COMPRISIING THE STEPS OF PROVIDING APHENYL-METHYL-SILICONE RESIN SPRAYING SAID SILICONE RESIN ON SAID FILMBETWEEN SAID END-CAPS TO PROVIDE A COATING THEREON, THEN BAKING SAIDRESISTOR ELEMENT WITH SAID COATING THEREON AT APPROXIMATELY 400*CFL FORABOUT ONE-HALF HOUR TO CURE SAID COATING, PROVIDING A TUBULAR CASINGHAVING AN INTERNAL DIAMETER SLIGHTLY LARGER THAN THE EXTERNAL DIAMETEROF SAID END-CAPS AND SLIDING SAID CASING OVER SAID END-CAPS WITH THEOPEN END PORTIONS OF SAID CASING EXTENDING BEYOUND SAID END-CAPS ANDWITH SAID END-CAPS FITTING CLOSELY WITHIN THE INTERNAL SURFACE OF SAIDCASING AND CONCENTRICALLY SUPPORTING SAID INTERNAL SURFCE OF SAID CASINGAROUND SAID FILM AT A CIRCUMFERENTIAL CLEARANCE SPACE THEREFROM, SEALINGTHE OPEN END PORTIONS OF SAID CASING TO ENCLOSE SAID RESISTOR ELEMENTTHEREIN, THE STEPS OF SEALING INCLUDING THE FOLLOWING METHOD , PROVIDINGA POWDER GLASS MATERIAL, A POLYSILOXAE RESIN, AND A SILICONE VARNISH,MIXING ABOUT FIFTY PERCENT BY VOLUME OF POWDER GLASS WITH ABOUT SAMEAMOUNT OF POLYSILOXANE RESIN TO OBTAIN A GLASS-SILICONE MIXTURE, MIXINGIN 20 PERCENT BY VOLUME OF SAID SILICONE VARNISH WITH ABOUT 80 PERCENTBY BY VOLUME OF SAID GLASS-SILICONE MIXTURE TO OBTAIN A SEALINGMATERIAL, APPLYING SAID SEALING MATERIAL IN SAID OPEN END PORTIONS OFSAID CASING AGAINST SAID CLOSLEY FITTING WND-CAPS SO AS TO BLOCK SAIDSEALING MATERIAL FROM CONTACTING SAID FILM TO ENCLOSE SAID RESISTORELEMENT THEREIN AIAR SETTING SAID SEALING MATERIAL FOR ABOUT TWENTY-FOURHOURS AND THEN BAKING SAID SEALING MATERIAL IN SAID END PORTIONS ATAPPROXIMATELY 400*F., FOR APPROXIMATELY ONE HOUR TO SEAL SAID RESISTORELEMENT IN SAID CASING